Conventional cellular smears collected on site by a clinician are unsuitable for automated screening using image analysis techniques due to variable thickness and cell overlap. A standardized and well-controlled specimen preparation procedure is necessary for quantitative analyses, such as in image cytometry. Dispersion of large cellular aggregates, optimal cellular preservation, and effective, efficient transfer of the cellular material to a slide are all requirements for quantitative analysis.
Advances in image analysis technology now make it possible to use an automated prescreening system for conventionally prepared cervical smears. One approach uses an automated system to identify a large proportion of normal cervical smears without human interaction. Other smears are referred to a cytotechnologist for standard manual screening. Although automation allows normal specimens to be completely removed from the manual workload, for such a system to be clinically useful, it should not increase the rate of false negative diagnoses.
Automated analysis of cytological material can be optimized by preparing monolayers of cells on the specimen slides. Previously, many methods have been used to produce a "monolayer" of cells on a specimen slide. A "monolayer" is defined as a substantially two-dimensional layer of uniformly distributed cellular material, predominantly made up of single cells and small clusters of cells, on a glass specimen slide or other substrate, without substantial folding or overlapping of cells. Preparation of a monolayer facilitates observation of cellular abnormalities, as compared to slides prepared using conventional smear techniques. Previous methods for producing monolayers include ultrasonic vibration, shearing with a rotor, syringing, forced filtration, centrifugation, sedimentation, and filter transfer. Unfortunately, these methods are unsuitable for automation, since detection is hindered by cell-folding, cell overlap, and distorted morphology.
An automated technique has been developed which is called "ThinPrep" (Cytec Corporation, Marlborough, Mass.). The technique is described by Hutchinson, M.L., et al., Anatomic Pathology, Vol. 96, No. 3, pp. 300-305 (1991), and involves using a disposable hollow cylinder, which contains a polycarbonate filter bonded to the base. The cylinder-filter assembly is inserted into a vial containing a cellular suspension. Cells are then dispersed by rotating the cylinder at high speed. A variable volume of suspension is drawn into the cylinder, thereby attracting cells onto the outer surface of the filter. After 40,000 to 50,000 cells are collected onto the filter, the cylinder is removed from the vial and inverted. The filtrate is then aspirated from the cylinder, and the cells, touch-transferred to a glass specimen slide. This procedure produces non-optimal cell dispersion cell overlap, and cell folding occurs. The automated device prepares slides one at a time and must complete a slide before going on to the next slide.